1. Field of the Invention
The present invention relates to a method and equipment for calibrating output levels of a waveform analyzing apparatus called a digitizer in which an input waveform is sampled periodically, each sampled value is converted into digital data and stored in a memory and the stored data is fast Fourier-transformed into a frequency domain to thereby produce frequency components of the input waveform and levels of the respective components or output levels of a waveform analyzing apparatus of simple type called digital voltmeter which can measure both a square wave signal and a sine wave signal.
2. Description of the Prior Art
FIG. 1 shows an example of a conventional method of calibrating a level of a waveform analyzing apparatus 12 provided in an analog test part of, for example, an IC (integrated circuit) tester.
According to the prior art, as shown in FIG. 1, a sine wave signal S of a predetermined frequency and a predetermined level is supplied from a calibrated standard signal generator 11 to a waveform analyzing apparatus 12, and a ratio between an output level of the waveform analyzing apparatus 12 at that time and the predetermined level of the input sine wave signal S is calculated as a correction coefficient for an output level in that frequency. Then, correction coefficients for a plurality of levels in a plurality of predetermined frequencies are calculated and stored in a memory of a test processor 13 as a table to thereby calibrate the level of the waveform analyzer 12. When the input waveform is analyzed by the waveform analyzer 12 after the calibration of the level of the waveform analyzer 12 has been finished, the test processor 13 reads the correction coefficients for frequencies and levels, which are analyzed results of the waveform analyzer 12, from the table and multiples the corresponding output levels with the correction coefficients to produce the multiplied output levels as measured results.
The aforenoted conventional method of calibrating the level of the waveform analyzing apparatus requires the expensive standard signal generator 11. Particularly, when the level of the waveform analyzing apparatus 12 provided in the analog test part of the IC tester is calibrated, the standard signal generator 11 is controlled by the test processor 13 of the tester via a GP-IB bus 22B. However, a transmission rate of the GP-IB bus 22 is low and also a plurality of levels of frequencies must be calibrated so that the standard signal generator 11 must be controlled a number of times, which takes a lot of time in calibrating the levels.